The field of the invention relates generally to the production of multicrystalline silicon and, more particularly, to the production of multicrystalline silicon by pulling a multicrystalline silicon ingot.
Conventional photovoltaic cells used for the production of solar energy utilize multicrystalline silicon. Multicrystalline silicon is conventionally produced in a directional solidification (DS) process in which silicon is melted in a crucible and directionally solidified in a separate or in the same crucible. The solidification of the ingot is controlled such that molten silicon is fed to the solidifying front of the casting. The multicrystalline silicon produced in such a manner is an agglomeration of crystal grains with the orientation of the grains relative to each other being random due to the high density of heterogeneous nucleation sites at the crucible wall. Multicrystalline silicon is generally the preferred silicon source for photovoltaic cells rather than single crystal silicon due to its lower cost resulting from higher throughput rates as compared to single crystal silicon production.
Multicrystalline silicon ingots grown by the DS process often have a reduced median grain size in the portion of the ingot near the crucible in which they are grown due to the many nucleation points at the crucible surface. This reduced median grain size and the solid-state diffusion of impurities from the crucible wall cause a reduction in the minority carrier lifetime in this region detrimentally affecting the resulting cells' efficiency. Accordingly, this portion of the ingot is generally not used to produce solar cells but rather is recycled by utilization in subsequent silicon charges.
Further, a substantial portion of a silicon ingot grown by the DS process near the top portion of the ingot cannot be used for device fabrication and cannot be recycled for further use due to high impurity content in the region. This high impurity content is caused by segregation of impurities in the melt as the ingot forms. As the ingot forms, the concentration of impurities in the melt generally increases due to the relatively small segregation coefficient. The top region of the ingot is typically the last region to solidify and contains the highest concentration of impurities.
A need exists for high throughput methods for producing muticrystalline silicon ingots that do not result in a reduction in the median grain size in peripheral portions of the resulting ingot and which generally do not generate significant regions in the ingot which cannot be used for device fabrication or recycle.